1. Field of the Invention
The present invention relates to a bar code reader for physical distribution system and a POS (point of sale) system etc.
2. Description of the Related Art
A conventional bar code reader of the above type is, for instance, disclosed in Japanese Unexamined Patent Publication No. Hei. 1-228076 (published Sep. 12, 1989). This conventional bar code reader will be described with reference to FIG. 1.
In FIG. 1, a lens 2 converges a laser beam 3 emitted from a He-Ne laser diode 1. A mirror 4 folds the converged laser beam 3 toward a polygon mirror 6 (described later). An aperture mirror 5, which has an aperture for passing the laser beam 3 folded from the mirror 4, is disposed between the mirror 4 and the polygon mirror 6. The polygon mirror 6, which is a rotary mirror shaped like a polygonal prism, reflects the laser beam 3 passed through the aperture mirror 5. Pattern forming mirrors 7 and 8 reflect the laser beam 3 deflected by the rotating polygon mirror 6 so as to form an X-shaped cross pattern 16 on a surface of an object 14 having a bar code symbol to be read that is conveyed on a conveyor 13. The bar code symbol 15 is scanned by the laser beam 3 in a crossed pattern (cross pattern 16), and reflected light, i.e., scattered light is reflected by the pattern forming mirrors 7 and 8, the polygon mirror 6, and then the aperture mirror 5. A condenser lens 9 condenses the scattered light as reflected by the aperture mirror 5. A photoelectric converter 10 photoelectrically converts the scattered light. A signal obtained by the photoelectric conversion is amplified by a video amplifier 11, and recognized by a decoder 12.
The cross pattern 16 is wide enough to cover the width of the conveyor 13 to allow the bar code symbol 15 to be read omnidirectionally. The decoder 12 recognizes data that is obtained by scanning the bar code symbol 15 from its starting end to stopping end at one time. In this case, the conveying intervals between the objects 14 to be read cannot be shorter than the scanning width of the cross pattern 16.
However, where a necessary scanning width, which is relatively large, is provided by one cross pattern 16, the conveying intervals between the objects 14 to be read become approximately equal to the scanning width, which means a very low conveying efficiency.
An attempt has been made in which the conveying intervals between the objects 14 to be read are reduced by half to improve the conveying efficiency. As shown in FIG. 2, the pattern forming mirrors 7 and 8 are halved into four pattern forming mirrors 7a, 7b, 8a and 8b, and their orientations are slightly changed to change reflecting directions of the laser beam 3. As a result, there are obtained two cross patterns 17 having a half width of the cross pattern 16. Since the width of the cross pattern is halved, the conveying intervals can also be halved.
However, where the scanning is performed by using two or more, smaller cross patterns to reduce the conveying intervals between the objects 14 to be read, straight scanning line portions necessarily become shorter. Therefore, there arises a problem that a large bar code symbol 15 that is not covered by the straight scanning line portions cannot be scanned at one time. In an example shown in FIG. 3, in which the scanning is performed by using two cross patterns, a bar code symbol whose width is larger than a straight line scanning length mn cannot be read by one scanning operation.
Further, since the fixed mirrors for pattern formation become shorter, a larger part of a video signal is occupied by shading portions. As a result, a correct reading operation can not be performed. In addition, the configuration of the optical system becomes very complex.
Another conventional bar code reader is disclosed in U.S. Pat. No. 5,028,770 (patented Jul. 2, 1991). In this bar code reader, a plurality of semiconductor laser light sources are driven so as to be switched for respective reflecting facet of a rotary polygon mirror. Scattered light beams reflected from a bar code symbol obtained with the respective light sources are added together, and introduced, as a single reflected light beam, to one photoelectric transducer. This bar code reader has advantages that the density of scanning patterns can be increased, and that the apparatus can be made thinner.